If you have Quicktime Pro, Movie Player lets you hit Command-F to present a movie in full screen mode. What I just found out, though, is that it presents the movie on whatever screen the mouse is in. Cool stuff.

As far as I can tell, you can only show full screen DVDs using DVD player on the 'main' screen (the one with the menu bar). You can move the menu bar to whichever screen you want using the Displays preference pane (appropriate tab only shows up if you have more than one monitor). On some machines (my wifes original TiBook 400) there doesn't seem to be enough video memory to run both screens in millions of colors while using dvd player.

There are two problems left though: one, if I remember correctly, dvd player blacks out both screens (I could be wrong about this) and two, if your menubar is hidden on the dvd player screen then it become kind of difficult to do your work.